Wiberg, Johan

KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630). (Brobyggnad)

2009 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

Today’s railway bridges are analysed in more detail for moving loads due to the increase in speeds and axle loads. However, these numerical analyses are very time consuming as they often involve many simulations using different train configurations passing at different speeds and many considerations to take into account. Thus, simplified bridge, track and train models are often chosen for practical and time efficient simulations.

The New Årsta Railway Bridge in Stockholm was successfully instrumented during construction. A simplified 3D Bernoulli-Euler beam element FE model of the bridge was prepared. The FE model was first manually tuned based on static load testing. The most extensive work was performed in a statistical identification of significantly influencing modelling parameters. Consequently, parameters to be included in an optimised FE model updating, with consideration also to synergy effects, could be identified. The amount of parameters included in the optimisation was in this way kept at an optimally low level. For verification, measurements from several static and dynamic field tests with a fully loaded macadam train and Swedish Rc6 locomotives were used. The implemented algorithms were shown to operate efficiently and the accuracy in static and dynamic load effect predictions was shown to be considerably improved.

It was concluded that the complex bridge can be simplified by means of beam theory and an equivalent modulus of elasticity, and still produce reliable results for simplified global analyses. The typical value of an equivalent modulus of elasticity was in this case approximately 25% larger than the specified mean value for the concrete grade in question.

The optimised FE model was used in moving load simulations with high speed train loads according to the design codes. Typically, the calculated vertical acceleration of the bridge deck was much lower than the specified allowable code value. This indicates that multispan continuous concrete bridges are not so sensitive to train induced vibrations and therefore may be suitable for high speed train traffic.

Finally, the relevant area of introducing the proposed FE model updating procedure in the early bridge design phase is outlined.

Abstract [en]

This paper presents a cost-effective method for the assessment of actual traffic loads on an instrumented bridge. The instrumentation of a newly constructed integral-type railway bridge in Stockholm (Sweden) is described. A complete “Bridge Weigh-in-Motion” (B-WIM) system, with axle detection and accurate axle-load evaluation, was implemented using only four concrete embedded strain transducers. A temporary accelerometer was attached to the edge beam of the bridge to evaluate the eigenfrequencies, predict possible wheel/rail defects, and check whether the acceleration limit value for ballast instability (as given in railway bridge design codes) is exceeded. The main objective of the monitoring project has been to increase the knowledge of actual traffic loads and their effect on railway bridges, through both measurements and numerical simulations. Some very early but representative results are presented, and the efficiency of the algorithms and usefulness of the monitoring program highlighted.

Abstract [en]

A new long-span prestressed railway bridge was instrumented to better understand and monitor its dynamic behaviour. The bridge is a unique and geometrically complex concrete structure with a very slender box girder section and a slab track system. This paper briefly describes the instrumentation used for monitoring the structural behaviour and focuses on investigating the dynamic characteristics of the bridge. The bridge’s dynamic properties were estimated using the output only stochastic subspace identification technique – for which the theory and analysis technique are briefly described – together with more traditional peak picking methods. Natural frequencies of the bridge were identified and verified from a previous study. The obtained frequencies and damping ratios are to be used in updating the developed finite element model. In addition, extreme bridge acceleration values from different train passages were collected and compared with the recommended limit value in bridge design codes.

Abstract [en]

The use of measurements allows for updated analysis models that better represent actual bridge behaviour. This paper investigated the possibility of using an equivalent modulus of elasticity in simplifying the modelling of a complex prestressed railway bridge. The study involved several static load tests with a fully loaded macadam train and Swedish Rc6 locomotives. Vertical deflection and axial strain measurements resulted in an equivalent modulus of elasticity based on a linear elastic Bernoulli-Euler beam element model. Calculated axial strain was refined as the Vlasov portion of the torsional moment due to constrained warping was considered. Relatively large variations in results were found but the identified modulus range verified the modulus found in previous modal analysis. It was concluded that the complex bridge can be simplified in means of beam theory and one single updating parameter, the equivalent modulus of elasticity, and still produce reliable results for simplified global analysis.

Abstract [en]

More accurate dynamic analysis of bridges is preferred instead of using the dynamic amplification factors in design codes to estimate the dynamic load effects. Additionally, the potentials of statistical methods are often overlooked and seldom used among structural engineers. However, for bridge engineers the parametric studies are still conservative and the finite element (FE) model results can often be misleading. In this paper, factorial experimentation in simulating railway bridge dynamics was exemplified. A two-level (2n) fractional factorial design was proposed and exemplified to screen the individual and joint effect of several modelling factors on the dynamic response of a specific railway bridge. The statistical theory proved to be relevant and meaningful, easy to implement but still very powerful. The concept of factorial experimentation was believed to provide the FE model updating engineer with important indications of the effects of several possible updating parameters.

Abstract [en]

The requirement of analyzing passing trains at high speeds in detail according to railway bridge design codes calls for time efficient and simplified FE models in some sense. This paper uses an optimized updating method based on load tests and statistically identified updating parameters. A large-scale simplified railway bridge FE model of a complex and continuous long span prestressed bridge is optimized for more time efficient and accurate load effect predictions. In addition, a benchmark test is presented to demonstrate the high potential of the adopted Nelder-Mead simplex optimization algorithm. The algorithm shows to operate efficiently and the accuracy in load effect predictions is considerably improved. High speed train model simulations are performed with the optimized FE model and more accurately predicted load effects are exemplified. The high potential FE model updating procedure is used traditionally, based on measurements, but the relevant area of introducing it in the early bridge design phase is discussed.